Not sure what the availability is of used stepside beds, or the cost in your area, but I'd tend to want to buy a used box and modify it. A decent short narrow box truck bed goes under $200 around here quite often, and could be narrowed, shortened, or reworked from there to get it looking like you want.
I like the idea of making a box from something else, but I think a garden cart bed at nearly $400, plus a lot of work would not be my choice. I also prefer the wider angled or flat top of a stock box.